1. Field of the Invention
The present invention relates to an islands-in-the-sea structure multifilament type plastic optical fiber having light transmitting islands as large as 100 to 10,000, which is characterized in that the light transmitted by this optical fiber is much brighter than the light transmitted by a multifilament type silica optical fiber, and a process for the preparation thereof.
2. Description of the Related Art
A multifilament type optical fiber comprising silica optical filaments having a diameter smaller than 200 .mu., which are arranged at a high degree of orientation, is capable of transmitting an image by light and is used as a fiber scope represented by a stomach camera, mainly in the field of medical instruments.
In the multifilament type silica optical fiber, the diameter of individual filaments can be made smaller than in the plastic type optical fiber, and therefore, development of a multifilament type silica optical fiber having an image element number as large as 10,000 or more has been advanced. However, this multifilament type optical fiber is seriously defective in that, since the fineness is very small and the resistance to bending is very low, the silica optical fiber is relatively easily broken by a bending operation during use and the broken part of the optical fiber causes an image defect.
Moreover, because of the properties of silica glass, the multifilament type optical fiber formed of silica optical filaments is inevitably rigid, and when the multifilament type optical fiber is used as an image scope, it is difficult to increase the bending angle, and thus the visual field that can be inspected is narrow.
Accordingly, development of a multifilament type plastic optical fiber comprising arranged many plastic optical filaments, which is not easily broken and can be bent more easily than the multifilament type silica optical fiber, has been attempted. For example, such attempts are disclosed in U.S. Pat. No. 3,556,635, Japanese Unexamined Patent Publication No. 56-39505, and European Patent No. 0.0207705-A2.
According to the invention disclosed in U.S. Pat. No. 3,356,635, a multifilament type plastic optical fiber is prepared by using an islands-in-the-sea conjugate spinning nozzle having a specific structure shown in the accompanying drawings. The islands-in-the-sea conjugate spinning spinneret used in this invention has a complicated structure such that core-forming pipes are vertically implanted in a sheath-forming orifice plate and sheath-forming pipes are vertically implanted in a sheath-forming orifice plate. Accordingly, dead spaces are formed in the portions of the respective nozzle ends on a sea-forming spinneret, and at the time of conjugate spinning, a hot molten polymer stagnates in these dead spaces and the polymer is thermally deteriorated. Therefore, it is difficult to prepare a multifilament type plastic optical fiber having a good light-transmitting property.
In the cross-section of the multifilament type plastic optical fiber prepared according to the invention disclosed in U.S. Pat. No. 3,556,635, islands exerting the light-transmitting function have a substantially rectangular cross-sectional shape, and these rectangular islands are regularly arranged in the longitudinal and lateral directions in the cross-section of the sea component, and the area occupied by the rectangular islands is less than 40% of the sectional area of the sea component. Accordingly, the multi-filament type optical fiber is defective in that a bright image cannot be transmitted with a good image-transmitting property.
Japanese Unexamined Patent Publication No. 56-39505 discloses an ultrafine multifilament type optical fiber having an islands-in-the-sea structure, and teaches that the number of islands is about 100,000 at largest and the cross-sectional area occupied by the islands is 50 to 98%. As disclosed in this unexamined patent publication, the cross-section of this multi-filament type plastic optical fiber has a circular structures and the cross-section of islands arranged in the central portion of this optical fiber's cross section has a circular structure and the image-transmitting property is good. However, the cross-section structures of the islands arranged in the peripheral portion of the multifilament type optical fiber are tetragonal to polygonal structure and the image-transmitting property of these islands is drastically reduced. Accordingly, the image-transmitting property of the multifilament type plastic optical fiber as a whole is unsatisfactory. This tendency becomes conspicuous as the cross-section structure of the multi-filament type optical fiber becomes close to a true circle and the ratio of the area occupied by the islands in the cross-section of the multifilament type optical fiber is increased, and the image-transmitting property becomes more unsatisfactory.
Accordingly, we previously proposed a process for preparing an excellent multifilament optical fiber in European Patent No. 0207705-A2. In this process, a conjugate spinneret is used for preparing an islands-in-the-sea multifilament type optical fiber, which comprises an orifice plate having many island-forming nozzle holes, which consists of core-forming orifice plate and sheath-forming orifice plate, an orifice plate having a sea-forming nozzle hole and a filament-gathering nozzle hole, wherein the nozzle hole of the orifice plate disposed just above the lowermost orifice plate, which is generally a sea-forming orifice plate, has a trumpet-shaped opening extending toward the lower end face thereof and many spun filaments are gathered in the filament-gathering nozzle. By this process, we succeeded in obtaining a multifilament type plastic optical fiber in which islands are arranged zigzag-stacked structure in the cross-section of the sea component.
According to this process, there can be obtained a multifilament type plastic optical fiber having a considerably good image-transmitting property, but although islands in the central portion of the multifilament type optical fiber's cross-section are arranged like zigzag-stacked structure, the cross-section structures of the islands are tetragonal to octagonal structures, and it is very difficult to increase the occupancy ratio of the cross-sectional area of the islands above 50% in the sectional area of the sea. Accordingly, a further improvement in the image-transmitting property is desired.